1. Baryonic Content in the Warm-Hot IGM at Low Redshift George Sonneborn FUSE Project Scientist, NASA/GSFC Mike Shull & Charles Danforth (Univ. Colorado), Warren Moos (Johns Hopkins Univ.) Ken Sembach (STScI), Blair Savage & Bart Wakker (U. Wisconsin), Todd Tripp (U. Mass), Ed Jenkins (Princeton), Chris Howk (Notre Dame), Jason Prochaska (Lick Obs), and others

2. Cosmic Mass-Energy Density WMAP &  CDM:  b =4.4% Baryon census at z~0 –Galaxies5-10% –21-cm H I~1% –Ly  - forest (T~10 4 K)~30% –WHIM (10 5-7 K)~50%? WHIM very low density (n~10 -4 ) not readily detectable in emission (~n 2 ) Absorption ~n more feasible, like Ly  forest

3. Far Ultraviolet Spectroscopic Explorer (1999-2007) 905 - 1187 Å,  v ~ 15 km/sec NASA/CNES/CSA PI-class mission –PI is Warren Moos, JHU >150 QSO/AGN observed at high S/N, t>6 Msec O VI 1032-38 Å resonance lines are most sensitive tracer of hot gas longward of Lyman limit –Traces gas at 10 5-6 K –Hot Milky Way halo and Galactic “corona” –Detected in IGM on many QSO sight lines (z<0.15) Limiting flux for IGM (high S/N) studies with FUSE was ~1x10 -14 erg/cm 2 /s/Å at 1030 Å

4. Far Ultraviolet Spectroscopic Explorer (1999-2007) 905 - 1187 Å,  v ~ 15 km/sec NASA/CNES/CSA PI-class mission –PI is Warren Moos, JHU >150 QSO/AGN observed at high S/N, t>6 Msec O VI 1032-38 Å resonance lines are most sensitive tracer of hot gas longward of Lyman limit –Traces gas at 10 5-6 K –Hot Milky Way halo and Galactic “corona” –Detected in IGM on many QSO sight lines (z<0.15) Limiting flux for IGM (high S/N) studies with FUSE was ~1x10 -14 erg/cm 2 /s/Å at 1030 Å

6. O VI IGM Results from FUSE WHIM predicted to be T~10 7 K –O VI best available tracer of WHIM (T~ 10 5-6 K) dN/dz ~ N -2 –Steep rise toward low N(OVI) –39 IGM absorbers; more on the way  (OVI) = 0.0031 +/- 0.0004 –~7% of baryons –Consistent with O VII-VIII being dominant ionization state in WHIM Much of the WHIM resides in structures at low column density Danforth & Shull (2005; 2007 in prep)

7. Metallicity of O VI absorbers ~9% Solar

8. Future Steps FUSE mission ended this summer due to failure of last reaction wheel –Plans to obtain data for >400 additional low N absorbers over next 2 years had to be abandoned –Analysis of existing FUSE QSO spectra in progress WHIM is major objective of Cosmic Origins Spectrograph (1150 - 3000 A) –SM4 in Sept. 2008 –O VI at z>0.14 Principal goal of Con-X mission is to study WHIM via X-ray lines (O VII and O VIII)